Download METABOLISM IN BACTERIA Microbial Metabolism Metabolism

METABOLISM IN BACTERIA
Microbial Metabolism
Metabolism refers the sum of biochemical reactions required for energy generation
and the use of energy to synthesize cellular materials.
The energy generation component is referred as catabolism and the build up of
macromolecules and cell organelles are referred as anabolism.
During
catabolism,
the
energy
is
changed
from
one
compound
to
another and finally conserved as high energy bonds of ATP.
ATP is the universal currency for energy. When energy is required for anabolism, it
may be sent as high energy bonds of ATP which has the value of 8 kcal per mole.
Based on the source of carbon, the microbes can be divided into two groups namely,
autotrophs and heterotrophs.
Autotrophs utilize CO2 as sole carbon source and
heterotrophs use organic carbon as sole carbon source.
I. Energy generation by heterotrophs
Heterotrophs use variety of carbon sources. Glucose is being the simple and wide
variety of microbes prefers it.
The glucose can be taken up by bacterium through diffusion
and can be readily utilized. There are three possible pathways available in bacteria to use
glucose. All these path ways are fermentative type and substrate level phosphorylation
occurs.
•
Embden-Meyerhof path way
•
Phosphoketolase path way
•
Entner – Doudoroff path way.
SUBSTRATE LEVEL PHOSPHORYLATION: (Fermentation)
The EMP pathway,
phosphoketolase pathway and ED pathway end with one or two
ATP synthesis by substrate level phosphorylation.
There won’t be any external source of
electron acceptor will come in these reactions.
A. Embden-Meyerhof path way
This is the path way of glycolysis most familiar and common to most of the
organisms. The path way is operated by yeast to produce alcohol and lactic acid bacteria to
produce lactic acid and several organic acids, gases, fatty acids, and alcohols. The path way
is as follows:
Glucose  2 pyruvate + 2 ATP + 2 NADH2
After pyruvate is formed, if the organism is a respirative type, the pyruvate will go
to Krebs cycle and if the organism is fermentative, the reduction process ends with
organic acids, alcohols etc.
A model fermentation:
After an intermediate product, a reduction takes place in
fermentation, whereas, if respiration, CO2 will be formed by complete oxidation through
Krebs’s cycle
(Note: After pyruvate, the reduction process leads to fermentation and complete oxidation
leads to respiration)
The Embden – Meyerhof path way can lead to a wide array of end products
depending on the path ways taken in the reductive steps after the pyruvate formation.
The following are some of the such fermentations:
Fermentation
End products
Model organism
Homolactic fermentation
Lactic acid
Lactobacillus
Mixed acid fermentation
Lactate,
acetate,
Enterobater
formate, succinate
Butyric acid fermentation
Butric acid, acetone
Clostridium
acetobutylicum
Propionic acid fermentation
Propionic acid
Propionibacterium
Alcohol fermentation
Ethanol
Saccharomyces
B. Phosphoketolase path way (Heterolactic path way)
The phosphoketolase path way is distinguished by the key cleavage enzyme
phosphoketolase, which cleaves pentose to glyceroldehyde 3 phosphate and acetyl
phosphate. The path way ends with ethanol and lactic acid. Ex. Lactobacillus, Leuconostoc.
The overall reaction is,
Glucose  1 lactate + 1 ethanol + 1 CO2 + 1 ATP
This path way is useful in the dairy industry for preparation of kefir (fermented milk),
yogurt, etc.
C. Entner – Doudoroff pathway
Only few bacteria like, Zymomonas mobilis employ the ED pathway. The path way is
as follows:
The overall reaction is
Glucose  2 ethanol + 2 CO2 + 1 ATP
The alcohol productivity of Zymomonas is higher than yeast because of this fermentative
pathway.
( Note : All the three pathways are end with 1 or 2 ATP by substrate level phosphorylation
by means fermentation)
OXIDATIVE PHOSPHORYLATION (Respiration)
If the organism is a respiratory type (that means complete oxidation of glucose), it
needs
four
essential
metabolic
components
for
their
respiration
and
oxidative
phosphorylation.
a. Tricarboxylic acid cycle (also known as citric acid cycle or Kreb’s cycle) The
pyruvate formed during glycolysis will be completely oxidized to 3 CO2 by the use of this
cycle. During oxidation of one pyruvate through TCA cycle, 4 NADH2, 1 FADH2 and 1 GTP
are produced along with 3 CO2.
b. A membrane and associated Electron Transport System (ETC) The electron
transport chain is a sequence of electron carriers transport the electrons to a terminal
electron acceptor.
across
the
During this flow of electron in the membrane, a proton motive force
membrane
leads
to
formation
ATP
(is
referred
as
electron
transport
phosphorylation).
c. An outside electron carrier: for aerobic respiration, O2 is the terminal electron acceptor
and reduced to H2O. This is normal for higher organisms. But in anaerobic bacteria, the
terminal electron acceptor may be of nitrite, nitrate, sulphate or carbon dioxide.
d. A membrane bound ATPase enzyme: The proton motive force developed during ETC
leads to formation of ATP by enzyme ATPase present in the membrane. (As in the diagram)
The table shows some aerobic and anaerobic respirations with specific examples:
Terminal
End product
Process name
Organism
O2
H 2O
Aerobic respiration
Streptomyces
NO3
NO2, N2
Denitrification
Pseudomonas denitrificans
SO4
S or H2S
Sulphate reduction
Desulfovibrio desulfuricans
Fumarate
Succinate
Anaerobic respiration
Escherichia
CO2
Methane (CH4)
Methanogenesis
Methanococcus
electron
acceptor
In aerobic organisms, the terminal electron acceptor will be of O2.
In some
anaerobic organisms, after the electron transport chain, instead of O2, some inorganic
compounds like sulphate, nitrate or some organic compounds like fumarate act as terminal
electron acceptor. Such type of respiration is referred as anaerobic respiration and the
normal O2 mediated respiration is referred as aerobic respiration. The above table shows
some anaerobic respiration with some terminal electron acceptors. The process is named
based on the compounds as sulphur reduction, denitrification and methanogenesis.
Energy generation by autotrophs
Autotrophs use CO2 as their sole carbon source.
There are two types such as
photoautotrophs and chemoautotrophs. Photoautotrophs use light as energy source and CO2
as carbon source. Chemoautotrops use chemicals (especially inorganic) as energy source
and CO2 as carbon source.
I. Energy and carbon assimilation by photoautotrphs: (Photoautotrophy)
Phototrophs use sunlight to produce ATP through phosphorylation, referred as
photophosphorylation. The phototrophs convert the light energy to chemical energy (ATP)
through the process called photosynthesis.
Photosynthesis is a type of metabolism in which catabolism and anabolism occur as
sequence.
The catabolic reaction (energy generating process) of photosynthesis is light
reaction in which the light energy is converted to chemical energy (ATP) and electrons or
reducing
powers
photosynthesis
is
(NADPH).
dark
The
reaction
in
anabolic
which
reaction
CO2 is
(carbohydrates), which is also called as CO2 fixation.
(macromolecule
converted
to
synthesis)
organic
of
molecules
For conversion of light energy to ATP, the bacteria possess light harvesting pigments.
They are chlorophyll a, carotenoids, phycobiliproteins (which are present in cyanobacteria)
and bacteriochlorophyll (which are present in purple sulphur bacteria). In bacteria, there are
two types of light reactions (conversion of light to ATP) and two types of CO2 fixation occur.
A. Light reaction (Photophosphorylation)
For photophosphorylation, light harvesting pigments, a membrane electron transport
chain, source of electron (electron donor) and ATPase enzymes are required. Two types of
photophosphorylations occur during photosynthesis. They are cyclic photophosphorylation
and non-cyclic photophosphorylation.

In plant and cyanobacteria, both cyclic and non-cyclic photophosphorylation occurs
whereas in purple bacteria, the cyclic photophosphorylation only occurs.

In plant and cyanobacteria, the electron source is water, by photolysis, H2O split into
+
H and O2 and during the process, O2 is evolved and referred as oxygenic photosynthesis

Since, the sulphur bacteria is an anaerobic bacterium, they use H2S instead of H2O as
electron donor. Since, there won’t be any O2 evolution during photosynthesis, referred as
anoxygenic photosynthesis.
Difference between plant and bacterial photosynthesis
organisms
Plant photosynthesis
Bacterial photosynthesis
plants, algae,
purple and green bacteria
cyanobacteria
type of chlorophyll
chlorophyll a
bacteriochlorophyll
absorbs 650-750nm
absorbs 800-1000nm
present
present
present
absent
Produces O2
Yes (Oxygenic)
No (Anoxygenic)
Photosynthetic electron donor
H 2O
H2S, other sulfur compounds
Photosystem I
(cyclic photophosphorylation)
Photosystem
II
(noncyclic
photophosphorylation)
or certain organic compounds
1. The oxygenic photophosphorylation
The end product of the light reaction is ATP, NADPH and O2. The ATP and NADPH,
the energy and electron sources thus produced were used for dark reaction.
2. The anoxygenic photo phosphorylation
The anoxygenic photo phosphorylation will take palce as in the image and the end
product of the light reaction is ATP, NADPH and Sulphur. The ATP and NADPH, the energy
and electron sources thus produced were used for dark reaction.
B. Dark reaction (CO2 fixation)
The dark reaction in which the ATP and NADPH were used as energy and electron
sources to fix the CO2 as carbohydrates.
The pathway involved in the dark reaction is
Calvin cycle, by which the CO2 is fixed as phosphoglyceic acid and lead to formation of
many sugars.
The enzyme RuBiSCO is the key enzyme for this process.
The following pathway shows the Calvin cycle and the formation of key monomers for
anabolic reactions such as hexose phosphate – polysaccharides; pyruvic acid – amino
acid and fatty acid; pentose phosphate – DNA and RNA.
A complete model of light and dark reaction of photosynthesis
Another way of CO2 fixation by phototrophs
In phototrophs, the electron and energy were derived form sunlight and carbon from
CO2 fixation through Calvin cycle. But some bacteria may derive electron and energy from
sunlight and fixes CO2 by some other path way, not the Calvin cycle.
The example is
Photosynthetic green bacteria (Chlorobium). They derive NADPH and ATP through cyclic
phosphorylation, but CO2 fixation is by reverse TCA cycle. Since TCA cycle is amphibolic
pathway (referring the cycle can operate in both the directions), it can also be used to fix
the carbon-di-oxide if operated reversely. The pathway is as follows:
Another way of CO2 fixation is by methanogens:
They use CO2 as terminal electron
acceptor and forms CH4 (methane). They also fix by acetyl CoA pathway for fixing CO2.
Synopsis:
Organism
Light reaction/ATP generation
Dark
reaction/CO2
fixation
Cyanobacteria
Cyclic
and
(Nostoc), plant
photophosphorylation
and alga
Oxygenic photosynthesis
Purple bacteria
Cyclic
(Chromatium)
photophosphorylation
and
non-cyclic
Calvin cycle
non-cyclic
Calvin cycle
non-cyclic
Reverse TCA cycle
Oxygenic photosynthesis
Green bacteria
Cyclic
and
(Chlorobium)
photophosphorylation
Oxygenic photosynthesis
II. Energy and carbon assimilation by Chemoautotrophs: (Chemoautotrophy)
Since the chemoautotrophs use inorganic chemicals for their energy and electron
source, they are referred as chemolithotrophs or chemolithotrophic autotrophs.
These organisms remove electron from an inorganic substance and put them through
electron transport chain for ATP synthesis (through electron transport phosphorylation). At
the same time, the electrons were also flow through reverse electron transport chain
and with the end product of NADPH.
through Calvin cycle.
These ATP and NADPH were used for CO2 fixation
These bacteria are obligate aerobic organisms.
Some examples of
the chemolithotrophs are as follows:
Groups of chemolithotrophs
Physiological group
Energy
Oxidized
source
end product
hydrogen bacteria
H2
H 2O
Alcaligenes, Pseudomonas
nitrifying bacteria
NH3
NO2
Nitrosomonas
nitrifying bacteria
NO2
NO3
Nitrobacter
sulfur oxidizing bacteria
H2S or S
SO4
Thiobacillus, Sulfolobus
iron oxidizing bacteria
Fe
2+
Fe
3+
Organism
Gallionella, Thiobacillus
The following diagram shows energy generation and CO2 fixation by different
chemolithotrophs: